OPEN ACCESS materials - Semantic Scholar...Key Laboratory of Comprehensive Utilization of Mineral...

Materials 2015, 8, 6105-6116; doi:10.3390/ma8095296OPEN ACCESS

materialsISSN 1996-1944

www.mdpi.com/journal/materials

Article

The Photoluminescent Properties of New Cationic Iridium(III)Complexes Using Different Anions and Their Applications inWhite Light-Emitting DiodesHui Yang, Guoyun Meng, Yayun Zhou, Huaijun Tang *, Jishou Zhao and Zhengliang Wang *

Key Laboratory of Comprehensive Utilization of Mineral Resources in Ethnic Regions,Joint Research Centre for International Cross-border Ethnic Regions Biomass Clean Utilization in Yunnan,School of Chemistry & Environment, Yunnan Minzu University, Kunming 650500, China;E-Mails: [email protected] (H.Y.); [email protected] (G.M.);[email protected] (Y.Z.); [email protected] (J.Z.)

* Authors to whom correspondence should be addressed; E-Mails: [email protected] (H.T.);[email protected] (Z.W.); Tel.: +86-871-6591-3043 (H.T.); Fax: +86-871-6591-0017 (H.T.).

Academic Editor: Harold Freeman

Received: 19 July 2015 / Accepted: 6 September 2015 / Published: 14 September 2015

Abstract: Three cationic iridium(III) complexes [Ir(ppy)2(phen)][PF6] (C1),[Ir(ppy)2(phen)]2SiF6 (C2) and [Ir(ppy)2(phen)]2TiF6 (C3) (ppy: 2-phenylpyridine, phen:1, 10-phenanthroline) using different anions were synthesized andcharacterized by 1H Nuclearmagnetic resonance (1HNMR), mass spectra (MS), Fourier transform infrared (FTIR)spectra and element analysis (EA). After the ultraviolet visible (UV-vis) absorption spectra,photoluminescent (PL) properties and thermal properties of the complexes were investigated,complex C1 and C3 with good optical properties and high thermal stability were used inwhite light-emitting diodes (WLEDs) as luminescence conversion materials by incorporationwith 460 nm-emitting blue GaN chips. The integrative performances of the WLEDsfabricated with complex C1 and C3 are better than those fabricated with the widely usedyellow phosphor Y3Al5O12:Ce3+ (YAG). The color rendering indexes of the WLEDs withC1 and C3 are 82.0 and 82.6, the color temperatures of them are 5912 K and 3717 K, and themaximum power efficiencies of them are 10.61 Lm¨W´1 and 11.41 Lm¨W´1, respectively.

Keywords: cationic iridium(III) complex; photoluminescence; white light-emitting diode;blue GaN chip

Materials 2015, 8 6106

1. Introduction

More and more interest is focused on white light-emitting diodes (WLEDs), because of their highefficiency, long lifetime, energy-saving and environmentally friendly properties [1–3]. At present, thecommercial WLEDs are mainly obtained by the combination of yellow phosphor Y3Al5O12:Ce3+ (YAG)with blue GaN-LED chips (λem « 460 nm). It is well known that the main emission wavelength of theYAG is in the greenish yellow region [4]. Thus, WLEDs fabricated with YAG have low color renderingindex (Ra) and high color temperature (Tc) because of the absence of red components in their spectra [5–7].In order to enhance the emission of YAG in red regions, YAG is optimized by doping with some rareearth ions (such as Eu3+, or Pr3+) [6–8]. Although optimized YAG exhibit slightly red emission, butthe yellow emission of the phosphors is obviously decreased. Hence, the development of new yellowphosphors for WLEDs based on blue LED chips is urgently needed.

Recently, many organic luminescent conversion materials have also been used in LEDs, such asorganic rare earth complexes [9–11], luminescent polymers [12–14] and small-molecule fluorescentdyes [15,16]. Cationic iridium(III) complexes with organic ligands composed of organic iridium(III)complex cation and inorganic acid anion (such as PF6´, ClO4´ and BF4´) have been widely applied inlight-emitting electrochemical cells (LECs) [17–19] and organic light-emitting diodes (OLEDs) [20–23],as well as used as highly efficient luminescent materials in metal-oxide/metal-organic frameworks(MOFs) for LEDs and chemical sensors [24–26] because of their excellent photochemical andphotophysical properties, such as high efficiency of 100% theoretical internal quantum efficiency,excellent color tenability via various ligands, short triplet state lifetimes, high thermal and photic stabilityand so on. These properties of cationic iridium(III) complexes meet the requirement of LEDs.

In this paper, three cationic iridium(III) complexes were synthesized with different anion sources, andtheir photoluminescence (PL) properties were investigated. Finally, the performances of WLEDs basedon them were investigated.

2. Experimental Section

2.1. Synthesis and Fabrication

All reagents and chemicals are of analytical grade and used as supplied without further purificationunless otherwise stated. The cationic iridium(III) complexes were synthesized according to our previouswork [20–22], as shown in Figure 1. The chloro-bridged dimer (ppy)2Ir(µ-Cl)2Ir(ppy)2 (643 mg,0.60 mmol, ppy:2-phenylpyridine) and 1,10-phenanthroline (phen, 237.6 mg, 1.2 mmol) were addedinto glycoland then kept at 150 ˝C in Ar atmosphere with stirring for 16 h. After being cooled toroom temperature, 10 mL 0.3 mol¨L´1 aqueous solution of ammonium salts NH4PF6, (NH4)2TiF6or (NH4)2SiF6 was added with stirring, respectively. After the counter ion-exchange reaction fromCl´ to PF6´, TiF62´ or SiF62´ [27], plentiful floccules precipitate appeared. The precipitate was filtered,washed with water and dried in vacuum. The crude product was purified by column chromatographyon silica gel with a mixture of CH2Cl2 and ethanol (volume ratio, 10:1) as eluent. All complexes werecharacterized by 1H Nuclear magnetic resonance (1HNMR), mass spectra (MS), elemental analysis (EA)and infrared spectra (IR). Yellow phosphor YAG was synthesized according to the reference [28]. The


stoichiometric mixtures of Y2O3, Al(OH)3 and CeO2 were ground and fired at 1300 ˝C for 8 h in reducingatmosphere (N2:H2 = 95:5).

The series of WLEDs were fabricated by coating the mixture of epoxy resin and iridium(III)complexes or YAG phosphors on GaN chips.

Materials 2015, 8 3

The series of WLEDs were fabricated by coating the mixture of epoxy resin and iridium(III) complexes or YAG phosphors on GaN chips.

Figure 1. Synthetic route and chemical structures of the cationic iridium(III) complexes.

2.2. Characterization

1HNMR spectra were recorded on a Bruker AV400 spectrometer operating at 400 MHz. Elemental analyses (EA) were performed on a Vario EL III Elemental Analysis Instrument. Mass spectra (MS) were obtained on a Bruker amaZon SL liquid chromatography mass spectrometer (LC-MS) with an electrospray ionization (ESI) interface using methanol as matrix solvent. Infrared spectra (IR) were recorded using a Fourier transform infrared spectrometer (IS10). Excitation and emission spectra were documented on a Cary Eclipse FL1011M003 (Varian, Palo Alto, CA, USA) spectrofluorometer, and the xenon lamp was used as excitation source. Thermogravimetric (TG) analysis was carried out up to 700 °C in N2 atmosphere with a heating speed of 10.0 K/min on a NETZSCH STA 449F3 thermogravimetric analyzer. The electroluminescent spectra of LEDs were recorded on a high-accuracy array spectrometer (HSP6000, HongPu Optoelectronics Technology Co. Ltd., Hangzhou, China).

[Ir(ppy)2(phen)][PF6] (C1), yellow solid, yield: 85%, 1HNMR (400 MHz, CD3OD, 25 °C, ppm): 8.77 (d, 2H, 3J = 8.0 Hz, ArH), 8.36 (d, 2H, 3J = 4.8 Hz, ArH), 8.30 (s, 2H, ArH), 8.13 (d, 2H, 3J = 8.0 Hz, ArH), 7.86–7.93 (m, 4H, ArH), 7.80 (t, 2H, 3J = 8.4 Hz, ArH), 7.45 (d, 2H, 3J = 5.6 Hz, ArH), 7.08 (t, 2H, 3J = 7.6 Hz, ArH), 6.95 (t, 2H, 3J = 8.0 Hz, ArH), 6.90 (t, 2H, 3J = 7.6 Hz, ArH), 6.41 (d, 2H, 3J = 7.6 Hz, ArH). FTIR (KBr, cm−1): 3452, 3131, 1693, 1657, 1609, 1580, 1476, 1123, 1068, 964, 847, 617, 560, 539, 517. ESI-MS (m/z, being shown in Figure S1): 681.1 [M-PF6]+. Element Anal. Calc. For C34H24F6IrN4P(%): C, 49.45; H, 2.93; N, 6.78; Found(%): C, 49.32; H, 2.86; N, 6.60.

[Ir(ppy)2(phen)]2SiF6 (C2), yellow solid, yield: 40%. 1HNMR (400 MHz, CDCl3, 25 °C, ppm): 8.93 (d, 4H, 3J = 8.0 Hz, ArH), 8.44 (s, 4H, ArH), 8.25 (d, 4H, 3J = 4.8 Hz, ArH), 7.88–7.93 (m, 8H, ArH), 7.72–7.73 (m, 8H, ArH), 7.31 (d, 4H, 3J = 5.6 Hz, ArH), 7.08 (t, 4H, 3J = 7.6 Hz, ArH), 6.97 (t, 4H, 3J = 7.6 Hz, ArH), 6.89 (t, 4H, 3J = 6.4 Hz, ArH), 6.40 (d, 4H, 3J = 7.6 Hz, ArH). FTIR (KBr, cm−1): 3453, 3133, 1694, 1657, 1461, 1390, 1350, 1264, 1123, 1068, 994, 954, 864, 821, 764, 618, 562, 539, 517.

Figure 1. Synthetic route and chemical structures of the cationic iridium(III) complexes.

2.2. Characterization

1HNMR spectra were recorded on a Bruker AV400 spectrometer operating at 400 MHz. Elementalanalyses (EA) were performed on a Vario EL III Elemental Analysis Instrument. Mass spectra (MS)were obtained on a Bruker amaZon SL liquid chromatography mass spectrometer (LC-MS) with anelectrospray ionization (ESI) interface using methanol as matrix solvent. Infrared spectra (IR) wererecorded using a Fourier transform infrared spectrometer (IS10). Excitation and emission spectra weredocumented on a Cary Eclipse FL1011M003 (Varian, Palo Alto, CA, USA) spectrofluorometer, and thexenon lamp was used as excitation source. Thermogravimetric (TG) analysis was carried out up to700 ˝C in N2 atmosphere with a heating speed of 10.0 K/min on a NETZSCH STA 449F3thermogravimetric analyzer. The electroluminescent spectra of LEDs were recorded on a high-accuracyarray spectrometer (HSP6000, HongPu Optoelectronics Technology Co. Ltd., Hangzhou, China).

[Ir(ppy)2(phen)][PF6] (C1), yellow solid, yield: 85%, 1HNMR (400 MHz, CD3OD, 25 ˝C, ppm): 8.77(d, 2H, 3J = 8.0 Hz, ArH), 8.36 (d, 2H, 3J = 4.8 Hz, ArH), 8.30 (s, 2H, ArH), 8.13 (d, 2H, 3J = 8.0 Hz,ArH), 7.86–7.93 (m, 4H, ArH), 7.80 (t, 2H, 3J = 8.4 Hz, ArH), 7.45 (d, 2H, 3J = 5.6 Hz, ArH), 7.08(t, 2H, 3J = 7.6 Hz, ArH), 6.95 (t, 2H, 3J = 8.0 Hz, ArH), 6.90 (t, 2H, 3J = 7.6 Hz, ArH), 6.41 (d, 2H,3J = 7.6 Hz, ArH). FTIR (KBr, cm´1): 3452, 3131, 1693, 1657, 1609, 1580, 1476, 1123, 1068, 964, 847,617, 560, 539, 517. ESI-MS (m/z, being shown in Figure S1): 681.1 [M-PF6]+. Element Anal. Calc.For C34H24F6IrN4P(%): C, 49.45; H, 2.93; N, 6.78; Found(%): C, 49.32; H, 2.86; N, 6.60.

[Ir(ppy)2(phen)]2SiF6 (C2), yellow solid, yield: 40%. 1HNMR (400 MHz, CDCl3, 25 ˝C, ppm): 8.93 (d,4H, 3J = 8.0 Hz, ArH), 8.44 (s, 4H, ArH), 8.25 (d, 4H, 3J = 4.8 Hz, ArH), 7.88–7.93 (m, 8H, ArH),7.72–7.73 (m, 8H, ArH), 7.31 (d, 4H, 3J = 5.6 Hz, ArH), 7.08 (t, 4H, 3J = 7.6 Hz, ArH), 6.97 (t, 4H,


3J = 7.6 Hz, ArH), 6.89 (t, 4H, 3J = 6.4 Hz, ArH), 6.40 (d, 4H, 3J = 7.6 Hz, ArH). FTIR (KBr, cm´1):3453, 3133, 1694, 1657, 1461, 1390, 1350, 1264, 1123, 1068, 994, 954, 864, 821, 764, 618, 562,539, 517. ESI-MS (m/z, being shown in Figure S1): 681.1 [1/2(M-SiF6)]+. Element Anal. Calc.For C68H48F6Ir2N8Si(%): C, 54.32; H, 3.22; N, 7.45; Found(%): C, 54.14; H, 3.53; N, 7.52.

[Ir(ppy)2(phen)]2TiF6 (C3), yellow solid, yield: 73%. 1HNMR (400 MHz, CDCl3, 25 ˝C, ppm):9.00 (d, 4H, 3J = 8.0 Hz, ArH), 8.49 (s, 4H, ArH), 8.25 (d, 4H, 3J = 4.8 Hz, ArH), 7.90–7.93 (m, 8H, ArH),7.72–7.74 (m, 8H, ArH), 7.32 (d, 4H, 3J = 6.0 Hz, ArH), 7.09 (t, 4H,3J = 7.2 Hz, ArH), 6.98 (t, 4H,3J = 8.0 Hz, ArH), 6.90 (t, 4H, 3J = 6.8 Hz, ArH), 6.40 (d, 4H, 3J = 7.6 Hz, ArH). FTIR (KBr, cm´1):3451, 3131, 1694, 1657, 1606, 1460, 1381, 1347, 1265, 1124, 1069, 994, 955, 865, 823, 762, 616, 562,539, 518. ESI-MS (m/z, being shown in Figure S1): 681.1 [1/2(M-TiF6)]+. Element Anal. Calc. ForC68H48F6Ir2N8Ti(%): C, 53.61; H, 3.18; N, 7.36; Found(%): C, 53.85; H, 3.45; N, 7.62.

3. Results and Discussion

3.1. UV-Vis Absorption Spectra

The UV-visible absorption spectra of the iridium(III) complexes in CH2Cl2 solution of 1.0 ˆ 10´5 mol¨L´1

at room temperature are shown in Figure 2. The intense absorption bands in the ultra-violet regionbetween 230 nm and 350 nm are ascribed to the spin-allowed 1π–π* transition of the ligands. Theweak absorption band from 350 nm extending to the visible region are overlapping absorption of1MLCT (metal-ligand charge-transfer), 1LLCT (ligand-to-ligand charge-transfer), 3MLCT, 3LLCT andligand-centered (LC) 3π–π* transitions [20,29]. The absorption of spin-forbidden 3MLCT, 3LLCT and3LCπ–π* mixing with higher-lying 1MLCT transitions exhibiting largish intensity is caused by the strongspin-orbit coupling endowed by the heavy iridium(III) atom [30,31]. Since all theabsorption spectra of thecomplexes are caused by the same organic iridium(III) complex cation [Ir(ppy)2(phen)]+, the absorptionspectra very much resemble one another, all of them have the same maximum absorption wavelengthpeaked at 267 nm. However, the absorption intensities of them at same wavelengths are different whichmeans the absorption is affected by different anions of PF6´, SiF62´ and TiF62´. The complex C3 withTiF62´ exhibits the maximum absorption intensity and that of the complex C2 with SiF62´ is the minimum.

Materials 2015, 8 4

ESI-MS (m/z, being shown in Figure S1): 681.1 [1/2(M-SiF6)]+. Element Anal. Calc. For C68H48F6Ir2N8Si(%): C, 54.32; H, 3.22; N, 7.45; Found(%): C, 54.14; H, 3.53; N, 7.52.

[Ir(ppy)2(phen)]2TiF6 (C3), yellow solid, yield: 73%. 1HNMR (400 MHz, CDCl3, 25 °C, ppm): 9.00 (d, 4H, 3J = 8.0 Hz, ArH), 8.49 (s, 4H, ArH), 8.25 (d, 4H, 3J = 4.8 Hz, ArH), 7.90–7.93 (m, 8H, ArH), 7.72–7.74 (m, 8H, ArH), 7.32 (d, 4H, 3J = 6.0 Hz, ArH), 7.09 (t, 4H,3J = 7.2 Hz, ArH), 6.98 (t, 4H, 3J = 8.0 Hz, ArH), 6.90 (t, 4H, 3J = 6.8 Hz, ArH), 6.40 (d, 4H, 3J = 7.6 Hz, ArH). FTIR (KBr, cm−1): 3451, 3131, 1694, 1657, 1606, 1460, 1381, 1347, 1265, 1124, 1069, 994, 955, 865, 823, 762, 616, 562, 539, 518. ESI-MS (m/z, being shown in Figure S1): 681.1 [1/2(M-TiF6)]+. Element Anal. Calc. For C68H48F6Ir2N8Ti(%): C, 53.61; H, 3.18; N, 7.36; Found(%): C, 53.85; H, 3.45; N, 7.62.

3. Results and Discussion

3.1. UV-Vis Absorption Spectra

The UV-visible absorption spectra of the iridium(III) complexes in CH2Cl2 solution of 1.0 × 10−5 mol·L−1 at room temperature are shown in Figure 2. The intense absorption bands in the ultra-violet region between 230 nm and 350 nm are ascribed to the spin-allowed 1π–π* transition of the ligands. The weak absorption band from 350 nm extending to the visible region are overlapping absorption of 1MLCT (metal-ligand charge-transfer), 1LLCT (ligand-to-ligand charge-transfer), 3MLCT, 3LLCT and ligand-centered (LC) 3π–π* transitions [20,29]. The absorption of spin-forbidden 3MLCT, 3LLCT and 3LCπ–π* mixing with higher-lying 1MLCT transitions exhibiting largish intensity is caused by the strong spin-orbit coupling endowed by the heavy iridium(III) atom [30,31]. Since all the absorption spectra of the complexes are caused by the same organic iridium(III) complex cation [Ir(ppy)2(phen)]+, the absorption spectra very much resemble one another, all of them have the same maximum absorption wavelength peaked at 267 nm. However, the absorption intensities of them at same wavelengths are different which means the absorption is affected by different anions of PF6−, SiF62− and TiF62−. The complex C3 with TiF62− exhibits the maximum absorption intensity and that of the complex C2 with SiF62− is the minimum.

250 300 350 400 450 500 550 6000.0

0.2

0.4

0.6

0.8

Abs

orpt

ion/

a.u.

Wavelength/nm

C1 C2 C3

Figure 2. UV-Vis absorption spectra of the cationic iridium(III) complexes in CH2Cl2 at 1.0 × 10−5 mol·L−1 at room temperature.

Figure 2. UV-Vis absorption spectra of the cationic iridium(III) complexes in CH2Cl2 at1.0 ˆ 10´5 mol¨L´1 at room temperature.


3.2. Photoluminescent Properties

The excitation and emission spectra of the cationic iridium(III) complexes in CH2Cl2 solutions at1.0 ˆ 10´5 mol¨L´1 at room temperature are shown in Figure 3. In addition, due to the same organiciridium(III) complex cation [Ir(ppy)2(phen)]+, three cationic iridium(III) complexes exhibit similarexcitation and emission spectra. For complexes C1, C2 and C3, the maximum excitation wavelengthsare 278 nm, 267 nm and 282 nm respectively, the maximum emission wavelengths are all 568nm. Ingeneral, for mixed-ligand cationic iridium(III) complexes, usually three excited states usually contributeto the observed light emission, those are 3LCπ–π*, 3MLCT and 3LLCT [32]. All complexes exhibitbroad and almost featureless emission spectra, which demonstrated that the emissive excited states havepredominantly 3LCπ–π* characters other than 3MLCT or 3LLCT [33].

Materials 2015, 8 5

3.2. Photoluminescent Properties

The excitation and emission spectra of the cationic iridium(III) complexes in CH2Cl2 solutions at 1.0 × 10−5 mol·L−1 at room temperature are shown in Figure 3. In addition, due to the same organic iridium(III) complex cation [Ir(ppy)2(phen)]+, three cationic iridium(III) complexes exhibit similar excitation and emission spectra. For complexes C1, C2 and C3, the maximum excitation wavelengths are 278 nm, 267 nm and 282 nm respectively, the maximum emission wavelengths are all 568nm. In general, for mixed-ligand cationic iridium(III) complexes, usually three excited states usually contribute to the observed light emission, those are 3LCπ–π*, 3MLCT and 3LLCT [32]. All complexes exhibit broad and almost featureless emission spectra, which demonstrated that the emissive excited states have predominantly 3LCπ–π* characters other than 3MLCT or 3LLCT [33].

300 400 500 600 700

0

100

200

300

400

Inte

nsity

/a.u

.

Wavelength/nm

C1 C2 C3

EmEx

Figure 3. Excitation (Ex, λem = 580 nm) and emission (Em, λex = 342 nm) spectra of the cationic iridium(III) complexes in CH2Cl2 at 1.0 × 10−5 mol·L−1 at room temperature.

Figures 4 and 5 are excitation and emission spectra of the solid powders of YAG and three cationic iridium(III) complexes. As shown in Figure 4, at emission wavelength of 565 nm, the complexes all exhibit similar broad excitation bands from 250 nm to 550 nm, and all of them have two peaks with the maximum excitation wavelengths around 337 nm and 444 nm respectively. The YAG has a main peak at 400–515 nm with the maximum excitation wavelengths of 460 nm. In other words, all of the above-mentioned phosphors can be well excited by 460 nm emitting blue GaN chip and white light can be obtained by combining light from the chip and from one of the phosphors. However, as shown in Figure 5, the emission bands of YAG and cationic iridium(III) complexes are different. The emission of YAG mainly contains greenish yellow light, so its combination with 460 nm emitting blue GaN chip will obtain cool white light, on the contrary, the emission of the cationic iridium(III) complexes covers yellow light and part of orange red light, which can be coated on 460 nm emitting blue GaN chip for obtaining warm white light. From the excitation and emission spectra, another piece of information can be obtained, that the emission intensity of cationic iridium(III) complex C3 with TiF62− is higher than that

Figure 3. Excitation (Ex, λem = 580 nm) and emission (Em, λex = 342 nm) spectra of thecationic iridium(III) complexes in CH2Cl2 at 1.0 ˆ 10´5 mol¨L´1 at room temperature.

Figures 4 and 5 are excitation and emission spectra of the solid powders of YAG and three cationiciridium(III) complexes. As shown in Figure 4, at emission wavelength of 565 nm, the complexes allexhibit similar broad excitation bands from 250 nm to 550 nm, and all of them have two peaks withthe maximum excitation wavelengths around 337 nm and 444 nm respectively. The YAG has a mainpeak at 400–515 nm with the maximum excitation wavelengths of 460 nm. In other words, all of theabove-mentioned phosphors can be well excited by 460 nm emitting blue GaN chip and white light canbe obtained by combining light from the chip and from one of the phosphors. However, as shown inFigure 5, the emission bands of YAG and cationic iridium(III) complexes are different. The emissionof YAG mainly contains greenish yellow light, so its combination with 460 nm emitting blue GaN chipwill obtain cool white light, on the contrary, the emission of the cationic iridium(III) complexes coversyellow light and part of orange red light, which can be coated on 460 nm emitting blue GaN chip forobtaining warm white light. From the excitation and emission spectra, another piece of information canbe obtained, that the emission intensity of cationic iridium(III) complex C3 with TiF62´ is higher than


that of the others, likely due to its higher excitation light absorption. The order of emission intensities isC3 > C1 > C2, which means that different anions in the complexes will affect their emission intensities.

Materials 2015, 8 6

of the others, likely due to its higher excitation light absorption. The order of emission intensities is C3 > C1 > C2, which means that different anions in the complexes will affect their emission intensities.

250 300 350 400 450 500 5500

50

100

150

2000

50

100

150

200

Wavelength/nm

C1 C2 C3

(b)

Inte

nsity

/a.u

.In

tens

ity/a

.u.

YAG(a)

Figure 4. Excitation spectra (λem = 565 nm) of YAG (a) and the cationic iridium(III) complexes (b) powders.

450 500 550 600 650 700 750

0

50

100

150

2000

100

200

300

(b)

wavelength/nm

C1 C2 C3

Inte

nsity

/a.u

.In

tens

ity/a

.u.

YAG(a)

Figure 5. Emission spectra (λex = 444 nm) of YAG (a) and the cationic iridium(III) complexes (b) powders.

Figure 4. Excitation spectra (λem = 565 nm) of YAG (a) and the cationic iridium(III)complexes (b) powders.

Materials 2015, 8 6

of the others, likely due to its higher excitation light absorption. The order of emission intensities is C3 > C1 > C2, which means that different anions in the complexes will affect their emission intensities.

250 300 350 400 450 500 5500

50

100

150

2000

50

100

150

200

Wavelength/nm

C1 C2 C3

(b)

Inte

nsity

/a.u

.In

tens

ity/a

.u.

YAG(a)

Figure 4. Excitation spectra (λem = 565 nm) of YAG (a) and the cationic iridium(III) complexes (b) powders.

450 500 550 600 650 700 750

0

50

100

150

2000

100

200

300

(b)

wavelength/nm

C1 C2 C3

Inte

nsity

/a.u

.In

tens

ity/a

.u.

YAG(a)

Figure 5. Emission spectra (λex = 444 nm) of YAG (a) and the cationic iridium(III) complexes (b) powders.

Figure 5. Emission spectra (λex = 444 nm) of YAG (a) and the cationic iridium(III)complexes (b) powders.


3.3. Thermal Stability

High thermal stability is an essential requirement for WLEDs, since WLEDs are fabricated and workusually at a temperature even near (but not exceeding) 150 ˝C [34]. The thermal properties of threecationic iridium(III) complexes are characterized by thermogravimetry (TG), and shown in Figure 6. Atlow temperature, approximately from room temperature to 220 ˝C for C1, from room temperature to155 ˝C for C2, and from room temperature to 185 ˝C for C3 respectively, every complex has a littlemass loss of adsorptive water and organic solvent residues, about 2.0% for C1, 4.0% for C2 and 3.0%for C3 respectively. Due to the easily degradable property of SiF62´ (SiF62´Ñ SiF4Ò + 2F´) [35], thereis an obvious mass loss process between 200 ˝C and 300 ˝C (with a point of inflection at about 230 ˝C)on the TG curve of C2; however, C1 and C3 with stable anions of PF6´ and TiF62´ do not showsimilar mass loss. At relatively high temperature, above 295 ˝C for C1, 345 ˝C for C2, and 315 ˝Cfor C3 respectively, every complex shows a big mass loss caused by the loss of neutral auxiliary ligands(1,10-phenanthroline) [22,36]. The results of thermal and optical properties suggest that complex C1and complex C3 are suitable to be used in LEDs but complex C2 is unsuitable.

Materials 2015, 8 7

3.3. Thermal Stability

High thermal stability is an essential requirement for WLEDs, since WLEDs are fabricated and work usually at a temperature even near (but not exceeding) 150 °C [34]. The thermal properties of three cationic iridium(III) complexes are characterized by thermogravimetry (TG), and shown in Figure 6. At low temperature, approximately from room temperature to 220 °C for C1, from room temperature to 155 °C for C2, and from room temperature to 185 °C for C3 respectively, every complex has a little mass loss of adsorptive water and organic solvent residues, about 2.0% for C1, 4.0% for C2 and 3.0% for C3 respectively. Due to the easily degradable property of SiF62− (SiF62−→ SiF4↑ + 2F−) [35], there is an obvious mass loss process between 200 °C and 300 °C (with a point of inflection at about 230 °C) on the TG curve of C2; however, C1 and C3 with stable anions of PF6− and TiF62− do not show similar mass loss. At relatively high temperature, above 295 °C for C1, 345 °C for C2, and 315 °C for C3 respectively, every complex shows a big mass loss caused by the loss of neutral auxiliary ligands (1,10-phenanthroline) [22,36]. The results of thermal and optical properties suggest that complex C1 and complex C3 are suitable to be used in LEDs but complex C2 is unsuitable.

100 200 300 400 500 600 70060

70

80

90

100 C1 C2 C3

Mas

s / %

Temperature / oC

Figure 6. Thermogravimetric curves of the cationic iridium(III) complexes.

3.4. Fabrication and Performance of WLEDs

In order to investigate the potential application of these cationic iridium(III) complexes, a series of WLEDs were fabricated by coating these complexes (doped in epoxy resin at mass ratio of 1:30) on the 460 nm emitting blue GaN chips. The electroluminescent (EL) spectra of these LEDs devices are shown in Figure 7. Figure 7a illustrates the EL spectrum of single LED chip with the strongest emission peaked at ~460 nm. Figure 7b is the EL spectrum of the WLED using the mixture of YAG and epoxy resin (the ratio of mass is 1:3) under 20 mA current excitation. The broad band in blue region is due to the emission of GaN chip, and the greenish yellow emission is due to the emission of YAG. The performance of this WLED is listed in Table 1. The WLED based on YAG exhibits high Tc (7338 K) and low Ra (74.7). Figure 7c,d presents the EL spectra of the WLEDs based on the mixture of the cationic iridium(III) complexes and

Figure 6. Thermogravimetric curves of the cationic iridium(III) complexes.

3.4. Fabrication and Performance of WLEDs

In order to investigate the potential application of these cationic iridium(III) complexes, a series ofWLEDs were fabricated by coating these complexes (doped in epoxy resin at mass ratio of 1:30) on the460 nm emitting blue GaN chips. The electroluminescent (EL) spectra of these LEDs devices are shownin Figure 7. Figure 7a illustrates the EL spectrum of single LED chip with the strongest emission peakedat ~460 nm. Figure 7b is the EL spectrum of the WLED using the mixture of YAG and epoxy resin (theratio of mass is 1:3) under 20 mA current excitation. The broad band in blue region is due to the emissionof GaN chip, and the greenish yellow emission is due to the emission of YAG. The performance of thisWLED is listed in Table 1. The WLED based on YAG exhibits high Tc (7338 K) and low Ra (74.7).Figure 7c,d presents the EL spectra of the WLEDs based on the mixture of the cationic iridium(III)complexes and epoxy resin (the ratio of mass is 1:30). Some differences can be found in the EL spectra


of these WLEDs from Figure 7. Firstly, the yellow emission part in spectrum of WLED with complexC3 shows obvious red-shift compared with that of YAG. Besides, the ration of blue emission and yellowemission in Figure 7d is smaller than that in Figure 7b. These results indicate that the WLED fabricatedwith complex C3 share better performance than that with YAG. The related parameters of these WLEDsare also listed in Table 1. Among these WLEDs based on iridium(III) complexes, the WLED fabricatedwith C3 shows the strongest white light, and shows lower Tc (3717 K) and higher Ra (82.6) comparedwith those of WLEDs based on YAG and C1.Moreover, a little of complex C3 can share intense yellowemission excited by the emission of GaN chip, compared with YAG. Hence the complex C3 maybe findapplication in WLEDs.

Materials 2015, 8 8

epoxy resin (the ratio of mass is 1:30). Some differences can be found in the EL spectra of these WLEDs from Figure 7. Firstly, the yellow emission part in spectrum of WLED with complex C3 shows obvious red-shift compared with that of YAG. Besides, the ration of blue emission and yellow emission in Figure 7d is smaller than that in Figure 7b. These results indicate that the WLED fabricated with complex C3 share better performance than that with YAG. The related parameters of these WLEDs are also listed in Table 1. Among these WLEDs based on iridium(III) complexes, the WLED fabricated with C3 shows the strongest white light, and shows lower Tc (3717 K) and higher Ra (82.6) compared with those of WLEDs based on YAG and C1.Moreover, a little of complex C3 can share intense yellow emission excited by the emission of GaN chip, compared with YAG. Hence the complex C3 maybe find application in WLEDs.

Figure 7. EL spectra of several LEDs at 20 mA forward bias: (a) The original blue GaN chip without phosphor (b) Blue GaN chip and YAG as phosphor (c) Blue GaN chip and complex C1 as phosphor (d) Blue GaN chip and complex C3 as phosphor.

Table 1. Performances of LEDs under 20 mA current excitation.

LED Mass ratio of Phosphor

and Epoxy Resin Tc

(K) Ra

Luminous Efficiency (Lm/W)

CIE (x, y)

only blue GaN chip ‒ 100000 49.5 12.91 (0.13, 0.06) YAG and blue GaN chip 1:3 7338 74.7 14.61 (0.29, 0.35)

C1 and blue GaN chip 1:30 5912 82.0 10.61 (0.32, 0.35) C3 and blue GaN chip 1:30 3717 82.6 11.41 (0.40, 0.40)

400 500 600 7000.0

0.5

1.0

0.0

0.5

1.0

0.0

0.5

1.0

0.0

0.5

1.0

Wavelength/nm

GaN and C3(d)

GaN and C1(c)

GaN and YAG(b)

Inte

nsity

/a.u

.

GaN(a)

Figure 7. EL spectra of several LEDs at 20 mA forward bias: (a) The original blue GaN chipwithout phosphor (b) Blue GaN chip and YAG as phosphor (c) Blue GaN chip and complexC1 as phosphor (d) Blue GaN chip and complex C3 as phosphor.

Table 1. Performances of LEDs under 20 mA current excitation.

LEDMass ratio of Phosphor

and Epoxy ResinTc (K) Ra

LuminousEfficiency (Lm/W)

CIE (x, y)

only blue GaN chip - 100000 49.5 12.91 (0.13, 0.06)

YAG and blue GaN chip 1:3 7338 74.7 14.61 (0.29, 0.35)

C1 and blue GaN chip 1:30 5912 82.0 10.61 (0.32, 0.35)

C3 and blue GaN chip 1:30 3717 82.6 11.41 (0.40, 0.40)


4. Conclusions

Three cationic iridium(III) complexes [Ir(ppy)2(phen)][PF6] (C1), [Ir(ppy)2(phen)]2SiF6 (C2) and[Ir(ppy)2(phen)]2TiF6 (C3) with different anions were synthesized. Complex C1 and C3 exhibit goodoptical properties and high thermal stability; however, these properties of complex C2 are poor, probablydue to the easily degradable property and high water adsorption of SiF62´. Complex C1 and C3exhibit intense and broad greenish-yellow emission with broad excitation bands in blue region. TheWLEDs fabricated using complex C1 and C3 as luminescence conversion materials show good opticalperformances that are better than those of the widely used yellow phosphor YAG; therefore, these twocomplexes (especially complex C3) are considered to be good candidates for WLEDs.

Supplementary Materials

Supplementary materials can be accessed at: http://www.mdpi.com/1996-1944/8/9/6105/s1.

Acknowledgments

This work was supported by National Nature Science Foundation of China (No. 21262046 and21261027), Program for Innovative Research Team (in Science and Technology) in Universities ofYunnan Province (2011UY09) and Yunnan Provincial Innovation Team (2011HC008).

Author Contributions

Hui Yang, Guoyun Meng and Yayun Zhou performed the experiments; Hui Yang, Huaijun Tang andZhengliang Wang analyzed the data; and Hui Yang wrote the initial draft of the manuscript. Huaijun Tangand Zhengliang Wang designed and supervised the project, reviewed and contributed to the final revisedmanuscript. All authors contributed to the analysis and conclusion, and read the final paper.

Conflicts of Interest

The authors declare no conflict of interest.

References

1. Nakamura, S. III–V nitride based light-emitting devices. Solid State Commun. 1997, 102, 237–248.[CrossRef]

2. Schubert, E.F.; Kim, J.K. Solid-state light sources getting smart. Science 2005, 308, 1274–1278.[CrossRef] [PubMed]

3. Pimputkar, S.; Speck, J.S.; DenBaars, S.P.; Nakamura, S. Prospects for LED lighting.Nat. Photonics 2009, 3, 180–182. [CrossRef]

4. Lee, S.; Seo, S.Y. Optimization of yttrium aluminum garnet: Ce3+ phosphors for white light-emittingdiodes by combinatorial chemistry method. J. Electrochem. Soc. 2002, 149, J85–J88. [CrossRef]

5. Lin, C.C.; Liu, R.S. Advances in phosphors for light-emitting diodes. J. Phys. Chem. Lett. 2011,2, 1268–1277. [CrossRef] [PubMed]

http://dx.doi.org/10.1016/S0038-1098(96)00722-3http://dx.doi.org/10.1126/science.1108712http://www.ncbi.nlm.nih.gov/pubmed/15919985http://dx.doi.org/10.1038/nphoton.2009.32http://dx.doi.org/10.1149/1.1511755http://dx.doi.org/10.1021/jz2002452http://www.ncbi.nlm.nih.gov/pubmed/26295420


6. Jang, H.S.; Im, W.B.; Lee, D.C.; Jeon, D.Y.; Kim, S.S. Enhancement of red spectral emissionintensity of Y3Al5O12:Ce3+phosphor via Pr Co-doping and Tb substitution for the application towhite LEDs. J. Lumin. 2007, 126, 371–377. [CrossRef]

7. Wang, X.; Zhou, G.; Zhang, H.L.; Li, H.; Zhang, Z.; Sun, Z. Luminescent properties of yellowishorange Y3Al5´xSixO12´xNx:Ce phosphors and their applications in warm white light-emitting diodes.J. Alloys Compd. 2012, 519, 149–155. [CrossRef]

8. Kwak, H.H.; Kim, S.J.; Park, Y.S.; Yoon, H.H.; Park, S.J.; Choi, H.W. Photoluminescencecharacteristic of Ce3+-Eu3+ Co-doped Y3Al5O12 phosphor prepared by combustion method.Mol. Cryst. Liq. Cryst. 2009, 513, 106–113. [CrossRef]

9. He, P.; Wang, H.; Liu, S.; Shi, J.; Wang, G.; Gong, M. Effect of different alkyl groups atthe N-position on the luminescence of carbazole-based β-diketonate europium (III) complexes.J. Phys. Chem. A 2009, 113, 12885–12890. [CrossRef] [PubMed]

10. Shao, G.; Zhang, N.; Lin, D.; Feng, K.; Cao, R.; Gong, M. A new europium(III)-β-diketonatecomplex based on diphenylethyne as red phosphors applied in LED. J. Lumin. 2013, 138, 195–200.[CrossRef]

11. Wang, H.; He, P.; Liu, S.; Shi, J.; Gong, M. New multinuclear europium(III) complexes asphosphors applied in fabrication of near UV-based light-emitting diodes. Inorg. Chem. Commun.2010, 13, 145–148. [CrossRef]

12. Luo, Y.; Yan, Q.; Zhang, Z.; Yu, X.; Wu, W.; Su, W.; Zhang, Q. White LED based onpoly(N-vinylcarbazole) and lanthanide complexes ternary co-doping system. J. Photochem.Photobiol. A Chem. 2009, 206, 102–108. [CrossRef]

13. Yan, H.; Wang, H.; He, P.; Shi, J.; Gong, M. An efficient bonding-type Eu-containing copolymeras red phosphor applied in LED. Inorg. Chem. Commun. 2011, 14, 1065–1068. [CrossRef]

14. Jin, J.-Y.; Kim, Y.-M.; Lee, S.-H.; Lee, Y.-S. Synthesis of an acrylic copolymer bearing fluorescentdye pendants and characterization as a luminescence conversion material in fabrication of aluminescence conversion light-emitting diode. Synth. Met. 2009, 159, 1804–1808. [CrossRef]

15. Jin, J.-Y.; Kim, H.-G.; Hong, C.-H.; Suh, E.-K.; Lee, Y.-S. White light emission from a blue LED,combined with a sodium salt of fluorescein dye. Synth. Met. 2007, 157, 138–141. [CrossRef]

16. Jin, J.-Y.; Lee, H.-Y.; Lee, S.-H.; Ko, S.-B.; Zong, K.; Lee, Y.-S. InGaN/bivalent fluorescein saltluminescence conversion light-emitting diode: Stability and photochemical reaction. J. Lumin.2007, 127, 665–670. [CrossRef]

17. Costa, R.D.; Ortí, E.; Bolink, H.J.; Monti, F.; Accorsi, G.; Armaroli, N. Luminescent ionictransition-metal complexes for light-emitting electrochemical cells. Angew. Chem. Int. Ed. 2012,51, 8178–8211. [CrossRef] [PubMed]

18. Su, H.-C.; Chen, H.-F.; Fang, F.-C.; Liu, C.-C.; Wu, C.-C.; Wong, K.-T.; Liu, Y.-H.; Peng, S.-M.Solid-state white light-emitting electrochemical cells using iridium-based cationic transition metalcomplexes. J. Am. Chem. Soc. 2008, 130, 3413–3419. [CrossRef] [PubMed]

19. Su, H.-C.; Fang, F.-C.; Hwu, T.-Y.; Hsieh, H.-H.; Chen, H.-F.; Lee, G.-H.; Peng, S.-M.;Wong, K.-T.; Wu, C.-C. Highly efficient orange and green solid-state light-emitting electrochemicalcells based on cationic IrIII complexes with enhanced steric hindrance. Adv. Funct. Mater. 2007,17, 1019–1027. [CrossRef]

http://dx.doi.org/10.1016/j.jlumin.2006.08.093http://dx.doi.org/10.1016/j.jallcom.2011.12.158http://dx.doi.org/10.1080/15421400903195908http://dx.doi.org/10.1021/jp908416qhttp://www.ncbi.nlm.nih.gov/pubmed/19807149http://dx.doi.org/10.1016/j.jlumin.2013.01.020http://dx.doi.org/10.1016/j.inoche.2009.10.031http://dx.doi.org/10.1016/j.jphotochem.2009.05.023http://dx.doi.org/10.1016/j.inoche.2011.03.076http://dx.doi.org/10.1016/j.synthmet.2009.05.030http://dx.doi.org/10.1016/j.synthmet.2007.01.008http://dx.doi.org/10.1016/j.jlumin.2007.04.004http://dx.doi.org/10.1002/anie.201201471http://www.ncbi.nlm.nih.gov/pubmed/22887710http://dx.doi.org/10.1021/ja076051ehttp://www.ncbi.nlm.nih.gov/pubmed/18302372http://dx.doi.org/10.1002/adfm.200600372


20. Tang, H.; Li, Y.; Chen, Q.; Chen, B.; Qiao, Q.; Yang, W.; Wu, H.; Cao, Y. Efficient yellow—Greenlight-emitting cationic iridium complexes based on 1,10-phenanthroline derivatives containingoxadiazole-triphenylamine unit. Dyes Pigments 2014, 100, 79–86. [CrossRef]

21. Zhang, F.; Duan, L.; Qiao, J.; Dong, G.; Wang, L.; Qiu, Y. Solution-processedblue—Green organic light-emitting diodes based on cationic iridium complexes with1-pyridyl-3-methylimidazolin-2-ylidene-C,C20 as the ancillary ligand. Org. Electron. 2012, 13,1277–1288. [CrossRef]

22. Tang, H.; Li, Y.; Zhao, B.; Yang, W.; Wu, H.; Cao, Y. Two novel orange cationic iridium(III)complexes with multifunctional ancillary ligands used for polymer light-emitting diodes.Org. Electron. 2012, 13, 3211–3219. [CrossRef]

23. Fernández-Hernández, J.M.; Yang, C.-H.; Beltrán, J.I.; Lemaur, V.; Polo, F.; Fröhlich, R.; Cornil, J.;de Cola, L. Control of themutual arrangement of cyclometalated ligands in cationic iridium(III)complexes. synthesis, spectroscopy, and electroluminescence of the different isomers. J. Am.Chem. Soc. 2011, 133, 10543–10558. [CrossRef] [PubMed]

24. Sun, C.-Y.; Wang, X.-L.; Zhang, X.; Qin, C.; Li, P.; Su, Z.-M.; Zhu, D.-X.; Shan, G.-G.;Shao, K.-Z.; Wu, H.; et al. Efficient and tunable white-light emissionof metal—Organicframeworks byiridium-complex encapsulation. Nat. Commun. 2013, 4. [CrossRef] [PubMed]

25. Medina-Castillo, A.L.; Fernández-Sánchez, J.F.; Klein, C.; Nazeeruddin, M.K.;Segura-Carretero, A.; Fernández-Gutiérrez, A.; Graetzelc, M.; Spichiger-Kellerb, U.E. Engineeringof efficient phosphorescent iridium cationic complex for developing oxygen-sensitive polymericand nanostructured films. Analyst 2007, 132, 929–936. [CrossRef] [PubMed]

26. De Queiroz, T.B.; Botelho, M.B.S.; Fernández-Hernández, J.M.; Eckert, H.; Albuquerque, R.Q.;de Camargo, A.S.S. New luminescent host—Guest system based on an iridium(III) complex:Design, synthesis, and theoretical—Experimentalspectroscopic characterization. J. Phys. Chem. C2013, 117, 2966–2975. [CrossRef]

27. He, L.; Duan, L.; Qiao, J.; Wang, R.; Wei, P.; Wang, L.; Qiu, Y. Blue-emitting cationic iridiumcomplexes with 2-(1H-pyrazol-1-yl) pyridine as the ancillary ligand for efficient light-emittingelectrochemical cells. Adv. Funct. Mater. 2008, 18, 2123–2131. [CrossRef]

28. Ikesue, A.; Kinoshita, T.; Kammata, K.; Yoshida, K. Fabrication and optical properties ofhigh-performance polycrystalline Nd: YAG ceramics for solid-state lasers. J. Am. Ceram. Soc.1995, 78, 225–228. [CrossRef]

29. Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P.E.;Forrest, S.R.; Thompson, M.E. Highly phosphorescent bis-cyclometalated iridium complexes:Synthesis, photophysical characterization, and use in organic light emitting diodes. J. Am. Chem.Soc. 2001, 123, 4304–4312. [CrossRef] [PubMed]

30. Wong, W.-Y.; Zhou, G.-J.; Yu, X.-M.; Kwok, H.-S.; Lin, Z.-Y. Efficient organic light-emittingdiodes based on sublimable charged iridium phosphorescent emitters. Adv. Funct. Mater. 2007,17, 315–323. [CrossRef]

31. Yu, X.-M.; Kwok, H.-S.; Wong, W.-Y.; Zhou, G.-J. High-efficiency white organic lightemittingdevices based on a highly amorphous iridium(III) orange phosphor. Chem. Mater. 2006, 18,5097–5103. [CrossRef]

http://dx.doi.org/10.1016/j.dyepig.2013.07.029http://dx.doi.org/10.1016/j.orgel.2012.03.017http://dx.doi.org/10.1016/j.orgel.2012.09.035http://dx.doi.org/10.1021/ja201691bhttp://www.ncbi.nlm.nih.gov/pubmed/21598947http://dx.doi.org/10.1038/ncomms3717http://www.ncbi.nlm.nih.gov/pubmed/24212250http://dx.doi.org/10.1039/b702628ehttp://www.ncbi.nlm.nih.gov/pubmed/17710269http://dx.doi.org/10.1021/jp309873yhttp://dx.doi.org/10.1002/adfm.200701505http://dx.doi.org/10.1111/j.1151-2916.1995.tb08389.xhttp://dx.doi.org/10.1021/ja003693shttp://www.ncbi.nlm.nih.gov/pubmed/11457197http://dx.doi.org/10.1002/adfm.200600359http://dx.doi.org/10.1021/cm061030p


32. Lowry, M.S.; Hudson, W.R.; Pascal, R.A., Jr.; Bernhard, S. Accelerated luminophore discoverythrough combinatorial synthesis. J. Am. Chem. Soc. 2004, 126, 14129–14135. [CrossRef][PubMed]

33. Tamayo, A.B.; Garon, S.; Sajoto, T.; Djurovich, P.I.; Tsyba, I.M.; Bau, R.; Thompson, M.E.Cationic bis-cyclometalated iridium(III) diimine complexes and their use in efficient blue, green,and red electroluminescent devices. Inorg. Chem. 2005, 44, 8723–8732. [CrossRef] [PubMed]

34. Wang, H.; He, P.; Liu, S.; Shi, J.; Gong, M. A europium(III) organic ternary complex appliedin fabrication of near UV-based white light-emitting diodes. Appl. Phys. B 2009, 97, 481–487.[CrossRef]

35. Patil, K.C.; Secco, E.A. Metal halide ammines. III. Thermal analyses and calorimetry offluorosilicateammines and hydrates of bivalent metals. Infrared spectra of the ammines. Can.J. Chem. 1975, 53, 2426–2429. [CrossRef]

36. Mu, P.; Wang, R.; Zhao, L.; Zhu, Z.; Guo, Y. Studies on the non-isothermal kineticsof thermal decomposition of binuclear europium m-methyl benzoate ternary complex with1,10-phenanthroline. Thermochim. Acta 1997, 306, 77–80. [CrossRef]

© 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access articledistributed under the terms and conditions of the Creative Commons Attribution license(http://creativecommons.org/licenses/by/4.0/).

http://dx.doi.org/10.1021/ja047156+http://www.ncbi.nlm.nih.gov/pubmed/15506778http://dx.doi.org/10.1021/ic050970thttp://www.ncbi.nlm.nih.gov/pubmed/16296826http://dx.doi.org/10.1007/s00340-009-3638-1http://dx.doi.org/10.1139/v75-343http://dx.doi.org/10.1016/S0040-6031(97)00305-5

1. Introduction2. Experimental Section2.1. Synthesis and Fabrication2.2. Characterization

3. Results and Discussion3.1. UV-Vis Absorption Spectra3.2. Photoluminescent Properties3.3. Thermal Stability3.4. Fabrication and Performance of WLEDs

4. Conclusions

OPEN ACCESS materials - Semantic Scholar...Key Laboratory of Comprehensive Utilization of Mineral...

Documents

Transcript of OPEN ACCESS materials - Semantic Scholar...Key Laboratory of Comprehensive Utilization of Mineral...